1. Field of the Invention
This invention relates to a triode-type fluorescent display device, and more particularly to a fluorescent device which is to prevent a mesh section of a control electrode from producing thermal deformation due to impingement of electrons.
2. Description of the Prior Art
A fluorescent display device includes a box-like envelope which is evacuated to a high vacuum. In the envelope, filamentary cathodes for emitting electrons, mesh-like control electrodes for accelerating and controlling electrons emitted from the filamentary cathodes, and phosphor deposited anodes for emitting light due to impingement of electron are arranged. As an example of mounting the control electrode within the envelope, a spacer frame is generally used. In FIG. 10, a fluorescent display device using the spacer frame is illustrated. The spacer frame designated by the reference numeral 2 is formed into a size larger than a substrate 1 and includes various kinds of electrodes, such as, for example, cathode supports 4, cathode leads 4a, mesh fixing frames 5, control electrode leads 5a contiguous to the mesh fixing frames 5, and anode leads 6 each having a contact to be connected to a connection terminal provided on the substrate 1. These electrodes are integrally formed on the spacer frame 2 so as to provide an electrode assembly 3. As shown in FIG. 9, the spacer frame 2 further includes a mesh framework 10 which is provided with mesh sections 9 each comprising a mesh 7 and a mesh frame 8. The mesh framework 10 is superposed on the spacer frame 2 so that the mesh frame 8 of the mesh section 9 may be welded to the mesh fixing frame 5 at the several spots, and then the remaining of the mesh framework 10 is removed to provide control electrodes G. In addition, the electrode assembly 3 includes filamentary cathodes K stretchedly arranged on the cathode supports 4. The electrode assembly 3 thus constructed is positioned on the substrate 1, and then a casing 11 of a box-like lid shape is sealedly mounted on the substrate 1, while being heated to 450.degree.-550.degree. C. to cause melting of frit glass, thereby to form an envelope. Then, the leads 4a, 5a and 6 airtightly passing through sealing portions of the envelope and led out to an exterior of the envelope are separated from a frame section of the spacer frame 2.
In the fluorescent display device as described above, the leads 4a, 5a and 6 are to be led out through the sealing portions of the envelope. Accordingly, the leads 4a, 5a and 6 and the spacer frame 2 are usually made of 426 alloy (Ni: 42%, Cr: 6%, Fe: balance) which exhibits good conformability to sealing glass and has coefficient of thermal expansion close to that of sealing glass so that leakage through the sealing portions may be decreased. The mesh section 9 welded to the mesh fixing frame 5 of the spacer frame 2 is generally formed of a metal which is less expensive than 426 alloy, such as, for example, SUS 304, SUS 430 alloy or the like. 426, SUS 304, and SUS 430 alloys have coefficient of average thermal expansion as shown in the following table.
______________________________________ Coefficient of Average Range of Temperature Alloy Thermal Expansion (/.degree.C.) (.degree.C.) ______________________________________ SUS 304 17.3 .times. 10.sup.-6 30-200 SUS 430 10.4 .times. 10.sup.-6 30-200 426 Alloy 7.6 .times. 10.sup.-6 30-200 ______________________________________
As another example of mounting the control electrode within the envelope, Japanese Patent Publication No. 30654/80 discloses a method for fixing control electrodes directly on a substrate. In the fluorescent display device of this type, control electrodes each are adhesively fixed on a connecting terminal section of a glass substrate by means of a conductive adhesive consisting essentially of Ag and frit glass. The control electrode includes a mesh portion, a rising portion and a flange portion each formed of the same metal. The control electrode is adhesively fixed by means of the flange portion on the glass substrate.
In the envelope of the fluorescent display device using the spacer frame described above, electrons emitted from the cathodes K partially become a reactive current by impinging the mesh section 9 of each of the control electrodes G. At this time, kinetic energy of the electron is converted into heat which increases a temperature of the mesh section 9, particularly, the mesh 7, as high as 200.degree.-250.degree. C. during operation of the fluorescent display device. The heat is also transmitted to the spacer frame 2 on which the mesh section 9 having the mesh 7 is fixed. However, an increase in a temperature of the spacer frame 2 is limited to as low as 90.degree. C., because the mesh fixing frame 5 is integrally provided with the control electrode lead 5a which is led out to the exterior of the envelope. Further, as described above, the mesh section 9 is formed from the material which has coefficient of average thermal expansion larger than the mesh fixing frame 5 and the like which is formed of 426 alloy. Accordingly, the mesh section 9 fixed on the mesh fixing frame 5 at a normal temperature is subjected to thermal expansion larger than that of the spacer frame 2 during operation of the device. This causes the mesh 7 to be deformed in such a shape that a central portion thereof is projected toward the cathodes K or anodes A. If the thermal deformation of the mesh 7 is excessive, the control electrodes G contact with the cathodes K or anodes A. Even if the contact is avoided, deformation of the mesh 7 causes a variation of a distance between the control electrodes and the cathodes K, which results in a variation of density of an anode current, and makes luminescence of the displays uneven or causes flickering of the display.
In the fluorescent display device directly mounting the control electrode on the substrate, the control electrode formed of SUS 304 or SUS 430 alloy has coefficient of average thermal expansion larger than that of the glass substrate on which the control electrode is mounted. Accordingly, it is subjected to thermal expansion larger than that of the substrate, which causes the mesh to be deformed toward the cathodes or the anodes, as in the fluorescent display device using the spacer frame.
There have been several proposals for solving the above problems. However, these proposals are not satisfactory to prevent deformation of the control electrodes G.
One of these proposals is to devide a display pattern to reduce dimensions of each control electrode so that thermal deformation of each mesh may be decreased to prevent each control electrode from being contacted with other electrodes.
However, some display patterns are impossible to be devided. Also, the luminous display using control electrods decreased in dimensions results in lowering of display density.
Another proposal is made in Unexamined Japanese Utility Model Application Publication No. 96763/85 to prevent the meshes from being deformed or bulged toward a substrate by providing a part of meshes with a support pawl which is projected toward the substrate. However, it is disadvantageous in that the formation of the support pawl makes the manufacturing of fluorescent display device complicated and increase its manufacturing cost. Furthermore, the support pawl obstructs observation of the luminous display of the device.
Japanese Utility Model Publication No. 40523/83 discloses a method of mounting the control electrode by fixing mesh grids with respect to a spacer frame while being expanded by heating at an operating temperature of a display device so that tensile stress may be exerted on a mesh section at a normal temperature. However, it is extremely difficult to mount the mesh grids at the operating temperature which is normally above 250.degree. C., and also the spacer frame may be deformed.
A further proposal is made in Japanese Utility Model Publication No. 41635/83 to prevent deformation at the mesh section by providing a portion of the mesh section with a notch to reduce an area of control electrodes. However, this makes the manufacturing of fluorescent display device too complicated and increases its manufacturing cost.
In view of the foregoing, the inventors have proposed a mesh used in the fluorescent display device which is formed of a metal having coefficient of average thermal expansion smaller than that of 426 alloy from which a spacer frame is formed so that the spacer frame may be subjected to thermal expansion larger than the mesh when the device is operated. In an experiment where 42 alloy (Ni: 42%, Fe: balance), which is conventionally used for a lead frame of an IC or the like, coefficient of average thermal expansion of which is 4.0-4.7.times.10 .sup.-6 /.degree. C. at a temperature of 30.degree.-450.degree. C. is used for the mesh, the mesh is stretched beyond its elongation limit by the spacer frame which is larger in thermal expansion than the mesh and is broken during the sealing step in which the fluorescent display device is exposed to a high temperature of 450.degree.-550.degree. C. As shown in FIG. 7, difference in elongation percentage between 42 alloy and 426 alloy is increased as a temperature rises. Furthermore, elastic limit of metals generally decreases as an environmental temperature rises. In view of these facts, it is noted that the mesh made of 42 alloy is subjected to plastic deformation at a temperature (about 500.degree. C.) during the sealing step. Thus, the above problems are not satisfactorily solved, if the metal having the coefficient of average thermal expansion smaller than 426 alloy is used for the mesh.
Further, it is noted that the grid directly mounted on the glass substrate in the fluorescent display device is free from breakages at the sealing temperature of above 400.degree. C. if the coefficient of average thermal expansion of the grid is larger than the glass substrate, because tension is prevented from being applied to the mesh. Furthermore, it is free from breakage below the driving temperature of 250.degree. C. if the coefficient of average thermal expension of the grid is smaller than that of the glass substrate, because tension is applied to the mesh to decrease deformation of the grid.
In light of these facts, the inventors have found that the material for the mesh should have the following properties in order to solve the above problems:
(1) The material has a coefficient of average thermal expansion smaller than that of the glass substrate at a temperature within the range of room temperature through 250.degree. C., and preferably smaller than 426 alloy;
(2) At a temperature above 250.degree. C., the material has a coefficient of average thermal expansion close to that of 426 alloy at a temperature of 30.degree. C. through 400.degree. C.; and
(3) Inasmuch as the difference in elongation percentage between the material and 426 alloy exerts tension to the meshes, the difference in coefficient of average thermal expansion between both materials should be within an elastic limit of the material. Namely, the difference in elongation percentage between both materials is preferably at 0.1% or less.
FIG. 8 shows relationships between elongation percentage as a function of temperature in the mesh material which satisfies the above properties (1) to (3) in contrast with 426 alloy.